There are 2 kinds of "SPD".
There's the old/original style, which today is more often used on mountainbikes and recreational riding,
and the newer type, which is a very large triangular plate, used predominantly on road bikes (racing community, enthusiasts, and sunday cyclists who have the means to buy the high-end stuff but don't really need it).
Personally, I prefer the original (traditional) SPD type cleats, even for my track and road bikes. I use mountainbike-type shoes, that allow me to walk around, run my errands, without the clip-clap and the skating all over the floor.
Within the original type, there are 3 models: SH51, SH52, SH56.
From what I could gather,
SH51 requires pivoting your foot horizontally to disengage, but according to a youtube video I saw today, will also disengage if your foot rolls (such as in the natural instinctive movement when you want to remove your foot to the side, or during a fall)...
SH56 disengages either by pivoting horizontally, or pointing your toe downward and pulling up, or twisting to either side. The SH56 is the most beginner-friendly.
Later was introduced the
SH52. this one holds on a bit tighter than the SH51, and isn't as prone to releasing if you roll your foot to the side. This is more for expert mountainbikers, who don't want the foot to disengage when they're doing advanced stuff,
or expert road cyclists, who don't like that the cleats disengage too easily during hard acceleration, or steep uphills.